For example, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (unit cell). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, predetermined numbers of the unit cells and the separators are stacked together to form a fuel cell stack.
Normally, as a fuel gas supplied to the fuel cell, a hydrogen gas produced from a hydrocarbon based raw fuel by a reforming apparatus is used. In the reforming apparatus, after a reforming raw material gas is obtained from the hydrocarbon based raw fuel such as a fossil fuel, e.g., methane or LNG, the reforming raw material gas is subjected to steam reforming or partial oxidation reforming, autothermal reforming or the like to produce a reformed gas (fuel gas).
Normally, the city gas used as a raw fuel contains, in addition to methane (CH4), hydrocarbon of high carbon (C2+) such as ethane (C2H6), propane (C3H6), and butane (C4H10). In the case of using the hydrocarbon of high carbon as a fuel of a solid oxide fuel cell, hydrocarbon of C2+ should be removed by reforming. It is because the carbon may be precipitated in the fuel pipe or on the anode to degrade the cell performance undesirably. In this case, the water vapor needs to be supplied excessively.
In this regard, Japanese Patent Publication No. 2003-507860 discloses a fuel cell system as shown in FIG. 12. In the fuel cell system, reaction of hydrocarbon fuel of high carbon (C2+) and the water vapor is induced in a steam preliminary reformer 1 to produce a fuel fluid containing hydrogen and methane. The temperature in the steam preliminary reformer 1 is 500° C. or less. The methane accounts for 20% or more of the fuel in volume as measured on the wet basis. The fuel fluid and oxidant are supplied into a hot fuel cell 2 to reform the methane. By the reaction of the fuel fluid at the anode (not shown) and the reaction of the oxidant at the cathode (not shown), power generation is carried out.
However, in the conventional technique, according to the disclosure, though the temperature of the steam preliminary reformer 1 is 500° C. or less, it is extremely difficult to maintain the temperature at a certain level. Therefore, the temperature in the steam preliminary reformer 1 can not be maintained in the range of 300° C. to 400° C. at all times, and the S/C (steam/carbon) ratio cannot be low. When the temperature in the steam preliminary reformer 1 becomes high relative to the S/C ratio, precipitation (coking) of carbon occurs easily.
Therefore, the S/C ratio needs to be considerably high relative to the target temperature. Further, it is necessary to supply water excessively for the reforming reaction. Therefore, the capacity of the water supply power source such as a water pump needs to be large. As a result, the load of the fuel cell is large uneconomically.